Evidence that a-wave latency of the electroretinogram is determined solely by photoreceptors

Targeted deletion of Crb1/Crb2 in the optic vesicle models key features of leber congenital amaurosis 8

The Crb1 and 2 (Crumbs homolog 1 & 2) genes encode large, single-pass transmembrane proteins essential for the apicobasal polarity and adhesion of epithelial cells. Crb1 mutations cause degenerative retinal diseases in humans, including Leber congenital amaurosis type 8 (LCA8) and retinitis pigmentosa type 12 (RP12). In LCA8, impaired photoreceptor development and/or survival is thought to cause blindness during early infancy, whereas, in RP12, progressive photoreceptor degeneration damages peripheral vision later in life. There are multiple animal models of RP12 pathology, but no experimental model of LCA8 recapitulates the full spectrum of its pathological features. To generate a mouse model of LCA8 and identify the functions of Crb1/2 in developing ocular tissues, we used an mRx-Cre driver to generate allelic combinations that enabled conditional gene ablation from the optic vesicle stage. In this series only Crb1/2 double knockout (dKO) mice exhibited characteristics of human LCA8 disease: locally thickened retina with spots devoid of cells, aberrant positioning of retinal cells, severely disrupted lamination, and depigmented retinal-pigmented epithelium. Retinal defects antedated E12.5, which is far earlier than the stage at which photoreceptor cells mainly differentiate. Most remarkably, Crb1/Crb2 dKO showed a severely attenuated electroretinogram at the eye opening stage. These results suggest that human LCA8 can be modeled in the mouse by simultaneously ablating Crb1/2 from the beginning of eye development. Importantly, they also indicate that LCA8 is caused by malfunction of retinal progenitor cells during early ocular development rather than by defective photoreceptor-Muller glial interaction, a mechanism proposed for RP12.

Novel mutations in the gene for α-subunit of retinal cone cyclic nucleotide-gated channels in a Japanese patient with congenital achromatopsia

PURPOSE

To present the characteristics and pathology of a patient with congenital achromatopsia.

PATIENT AND METHODS

The patient was a 22-year-old Japanese woman who was 8 years old when she first visited our clinic. Comprehensive ophthalmic examinations including visual acuity measurements, perimetry, optical coherence tomography (OCT), fundus autofluorescence (FAF) imaging, electroretinography (ERG), and color vision tests were performed. Her genomic DNA was used as the template for the amplification of exons of five candidate genes for achromatopsia; CNGA3, CNGB3, GNAT2, PDE6C, and PDE6H, and the amplified products were sequenced. A missense mutation, found in the CNGA3, was studied both electrophysiologically and biochemically.

RESULTS

Her phenotype was typical of congenital complete achromatopsia. She was followed for 14 years, and her vision and fundus findings were stable. However, the scotopic ERG b-waves at age 22 were smaller than those at age 8, and her FAF images showed increased autofluorescence in both maculae. Genetic examinations revealed combined heterozygous mutations of c.997_998delGA and p.M424V in the CNGA3 gene. The homomeric channel consisting of the CNGA3 subunit with the p.M424V mutation had a weak cGMP-activated current in patch-clamp recordings. In heterologous expression analyses, the expression at the cell surface of the mutant CNGA3 subunit was about 28 % of the wild type.

CONCLUSIONS

The two novel mutations found in the CNGA3 gene, c.997_998delGA and p.M424V, can cause complete achromatopsia. The vision of the patient was stationary until the third decade of life although the FAF was altered at the age of 22 years.

The reflection of retinal light response information onto the superior colliculus in the rat

BACKGROUND

The functional principles of mediation of retina-encoded visual information through the optic nerve to the superior colliculus (SC) of the contralateral brain hemisphere were investigated in non-drugged and unrestrained albino rats by considering the following issues: (1) the type of information transmitted, (2) the response components of the retina and SC involved in encoding the transmitted information, and (3) the timing of related processes.

METHODS

The field potential responses for different intensities of flashes, under different background illuminations, were simultaneously recorded from the sclera area of the eye and the optic layer of the contralateral SC.

RESULTS

It was found that the b-wave crest of the retinal electroretinogram (ERG) and the peak-1 or peak-2 of the SC correlate by their amplitude, while the a-wave trough of the retinal ERG and the peak-1 of the SC correlate by their latency. The values of these mutually correlating response components were invariably determined by the given light response bias of the retina (photoreceptors), the change in the photon flux of the light stimulus and, obviously, the change in the wavelength of the light stimulus. The a-wave trough, peak-1, b-wave crest and peak-2 were invariably induced in this time-order.

CONCLUSIONS

The data suggest that the information properties of (a) intensity, (b) presentation time and, obviously, (c) colour of the light stimulus, such as are shed on the retina, and information about the light response bias of the retina are mediated correlatively and quantitatively to the cell network system of the SC through the optic nerve. These processes must happen during the a-to-b-wave phases of the ERG. The data indicate that the random-type variations in the activity of the related cellular systems may actually be harnessed in mediating the defined information properties of the visual stimulus from the retina to the SC of the brain through the optic nerve. This study shows a method of measuring the function of the optic nerve.

Time-Dependent Course of Electroretinograms in the Spontaneous Diabetic Goto-Kakizaki Rat

PURPOSE

This study evaluates the relevance to human retinopathy of electroretinograms (ERGs) from the spontaneously diabetic Goto-Kakizaki (GK) rat.

METHODS

Starting from 4 weeks of age, we obtained ERGs every 4 weeks from six GK rats and seven Wistar (control) rats, and from two GK and two Wistar rats at 14 days of age. The a-wave, b-wave, and oscillatory potentials (OPs) were recorded after stimulation with a single bright flash. We compared the amplitudes and implicit times and measured a-wave latencies to evaluate photoreceptor function.

RESULTS

The amplitudes of the a-wave, b-wave, and OPs (O1 and O2) of the GK rats were reduced between 4 and 48 weeks of age. The time-dependent courses of change in a-wave, b-wave, and O2 amplitude did not differ between the two groups. The a-wave latencies in GK rats were significantly prolonged, but not the implicit times of OPs. At 14 days of age, the a-wave amplitudes were significantly smaller in GK than in Wistar rats.

CONCLUSION

Functional abnormalities of photoreceptors might be induced by inheritable degeneration at an early age in the GK rat. Although hyperglycemia would cause retinal hypoxia, it would not be severe enough to disturb the generation of OPs.